1. Field of the Invention
This invention relates generally to marine drilling apparatus and methods for deep water operations. Specifically the invention relates to a method and apparatus for preventing the displacement of drilling mud by formation gas in the drilling riser thereby preventing the collapse of the riser due to the external pressure of the seawater.
2. Description of the Prior Art
The oil industry has had great success in pushing back the water depth frontiers for exploratory drilling offshore. Many technical challenges have been successfully met in the course of this effort. Two areas that are particularly noteworthy in this respect are the extension of the capability of marine risers and the development of new well control techniques. However, the problem of hydrostatic collapse of a marine riser in deep water has remained unsolved in this emerging technology.
On occasion a deepwater marine riser has collapsed at its lower segments. When the mud column in the riser annulus about a drill pipe has been displaced by formation gas, the resulting low pressure in the riser annulus has rendered the pressure differential (.DELTA.P) across the pipe wall so great that the collapse strength of the pipe has been exceeded. To combat such an occurrence, some deepwater risers have been equipped with riser fill valves which are intended to open the riser annulus to seawater influx before critical .DELTA.P has been reached. Typically, such valves are fitted with controls that sense .DELTA.P changes and automatically open the valve. A manual override is usually provided. Because of the unpredictability of the formation pressures such inflexible means of controls have been uncertain and risky. At best, such apparatus provides only one opportunity to restore equilibrium with critical dependence on the density of seawater. If the kinetic energy of the flowing fluid is sufficiently high, entering seawater may be ejected from the top of the riser.
3. Identification of Objects of the Invention
It is therefore an object of the invention to provide a well control system and apparatus to control the well when formation gas displaces drilling mud in a deepwater riser.
It is another object of the invention to provide a marine riser well control apparatus and method whereby formation gas which displaces drilling fluid in a deepwater riser may be circulated out via the drilling rig choke manifold while maintaining pressure of control over the well and the deepwater riser.